Add bridge from Throwable to E for ApplicativeError

[morgen-peschke]

Interacting with code that can throw exceptions can be awkward when working with ApplicativeError if E isn't Throwable, because the ApplicativeError#catch* methods don't really have a way to bridge into F unless you know what F is.

You can absolutely make it work, you just can't really leverage the ApplicativeError#catch* helpers, which is a shame because they're really handy, and I think it would be really straightforward to offer alternatives that are more generally applicable.

I propose adding parallel methods which would allow adapting a Throwable to an E. For example, ApplicativeError#catchNonFatalAs could look like this:

  /**
   * Often E can be created from Throwable. Here we try to call pure or 
   * catch, adapt into E, and raise.
   *
   * Exceptions that cannot be adapted to E will be propagated 
   */
  def catchNonFatalAs[A](adaptIfPossible: Throwable => Option[E])(a: => A): F[A] =
    try pure(a)
    catch {
      case NonFatal(e) => adaptIfPossible(e).map(raise(_)).getOrElse(throw e)
    }

Similar methods could be created for ApplicativeError#catchNonFatalEval and ApplicativeError#catchOnly.

[armanbilge]

This is really interesting!

Exceptions that cannot be adapted to E will be propagated

This doesn't seem right to me. Exception-throwing code should ideally never be written outside of a catchNonFatal; pure code that uses this method (or any method in Cats) should not throw exceptions.

It seems to me what you actually want is maybe something like F[G[A]], with ApplicativeError[G, E] and ApplicativeThrow[F]. That way non-adaptable exceptions can be safely handled in the F effect.

In which case I think you want something like this?

F.catchNonFatal(doTheThing()).map(G.pure).recover {
  case ex: MyAdaptableException => G.raiseError(whatever)
}

Maybe a dedicated method/syntax could make that a bit neater. I'm not entirely sure where it could live.

---

While writing this, I noticed catchOnly does the same propagating thing. So maybe there's precedent for this, then, but personally I don't like it.

cats/core/src/main/scala/cats/ApplicativeError.scala

Lines 263 to 267 in bf216a4

	/**
	* Evaluates the specified block, catching exceptions of the specified type. Uncaught exceptions are propagated.
	*/
	def catchOnly[T >: Null <: Throwable]: CatchOnlyPartiallyApplied[T, F, E] =
	new CatchOnlyPartiallyApplied[T, F, E](this)

[morgen-peschke]

If I understand the existing catchOnly, the intent is for anything that isn't caught to be considered a fatal error, and not something the application logic should be expected to handle, so I've followed a similar convention to minimize surprises.

I could be missing something though. The local helper this grew out of did originally require the adapting function to be total, so I'm certainly am not opposed to this approach.